A 170g billiards ball moving East at 28m/s, collides head on with a 240g cue ball moving at a velocity of 19m/s West. After the collision the 170g ball is moving West at 11m/s. What was the change in momentum of the 170g ball? The 240g ball? The closest I got to the answer is 6.3kgm/s. But it's 6.6kgm/s. I also don't get how change in momentum is positive for the 170g but the 240g ball is also the same value in the opposite direction. I get that the cons. of momentum is based on Newton's 3rd Law, but I'm not quite grasping the concept.
Well, let's look at the situation. Let a positive velocity mean moving east and a negative velocity mean moving west. The momentum of the 170g ball prior to the collision is 170*28. The momentum of the 170g ball after the collision is 170*(-11). Therefore the change in momentum is 170*28-170*(-11) = 6630 gm/s = 6.630 kg m/s. In order to find the change in momentum of the 240g ball, there's no need to find its velocity after the collision. Since we know momentum is conserved (and since there are only two objects in the system), a change in momentum in one ball must be the change in momentum of the other. This is to ensure that the net momentum of the system is zero. In this particular problem, the change in momentum of the 240g ball is -6.630 kg m/s. Notice how when you sum the changes in momentum of the two balls, you get 0. That means that the system as a whole did not have a momentum change and momentum is conserved.
Ugh oh x_x I added the momentum of both balls *lol stupid* How bout questions with gun and bullet? {1D, not 2D} Do you treat it as one object or what? The gun is initially at rest, but you account for the mass of the bullet already in the gun right? Then after it's fired, it becomes two different things? Actually what i'm thinking now, coming back to edit this, is that before the collision they're "stuck" together, and they get unstuck, so it's sorta like an inelastic question backwards?
Right, in this type of problem, consider the momentum of the system. The momentum of the gun+bullet before firing = momentum of gun + momentum of bullet after firing. Ehhh, don't think of it as backwards inelastic. That'll get confusing and you'll get the wrong answer